docs: def to_dict(self, nested=False):
          """Return dict object with model's data.
  
          :param nested: flag to return nested relationships' data if true
          :type: bool
          :return: dict
          """
          result = dict()
          for key in self.columns:
              result[key] = getattr(self, key)
  
          if nested:
              for key in self.relations:
                  obj = getattr(self, key)
  
                  if isinstance(obj, SerializeMixin):
                      result[key] = obj.to_dict()
                  elif isinstance(obj, Iterable):
                      result[key] = [o.to_dict() for o in obj]
  
          return result